Table Of Contents
Table Of Contents

Add JavaScript for Dynamic Control Behavior

Last Updated: March 2020

In this tutorial you will add dynamic behavior to the dataset controls created in the previous step using JavaScript. The following topics will be reviewed in this tutorial:

  • JavaScript Fundamentals

  • JavaScript Closure Module Method

  • Using JavaScript with Tethys Gizmos


0. Start From Previous Solution (Optional)

If you wish to use the previous solution as a starting point:

git clone
cd tethysapp-earth_engine
git checkout -b dataset-controls-solution dataset-controls-solution-3.1

1. Stub out new JavaScript file

In this step you'll learn how to create a JavaScript module using the closure method. For more on JavaScript closures see: JavaScript Closures - MDN. The basic module developed in this step will contain variables for storing the current value of each control and it will also read in the EE_PRODUCTS data you embedded in the template in the previous tutorial.

  1. Create a new JavaScript file at public/js/gee_datasets.js with the following contents:

var GEE_DATASETS = (function() {
    // Wrap the library in a package function
    "use strict"; // And enable strict mode for this library

    *                      MODULE LEVEL / GLOBAL VARIABLES
    var MODIS = 'modis',
        SENTINEL = 'sentinel',
        LANDSAT = 'landsat',

    var public_interface;

    // Selector Variables
    var m_platform,

    *                    PRIVATE FUNCTION DECLARATIONS
    // Dataset Select Methods
    var bind_controls, update_product_options, update_sensor_options, update_date_bounds, collect_data;

    // Dataset Select Methods
    bind_controls = function() {};

    update_sensor_options = function() {};

    update_product_options = function() {};

    update_date_bounds = function() {};

    collect_data = function() {};

    *                            PUBLIC INTERFACE
    public_interface = {};

    *                  INITIALIZATION / CONSTRUCTOR
    $(function() {
        // Initialize Global Variables

        // Initialize Constants
        EE_PRODUCTS = $('#ee-products').data('ee-products');
        INITIAL_START_DATE = m_start_date = $('#start_date').val();
        INITIAL_END_DATE = m_end_date = $('#end_date').val();

        // Initialize members
        m_platform = $('#platform').val();
        m_sensor = $('#sensor').val();
        m_product = $('#product').val();
        m_reducer = $('#reducer').val();

    return public_interface;

}()); // End of package wrapper


The lines that define empty functions (e.g.: bind_controls = function() {};) are method stubs that will be implemented in the following steps.

  1. Include the new gee_datasets.js script in the templates/earth_engine/home.html template:

{% block scripts %}
  {{ block.super }}
  <script src="{% static 'earth_engine/js/gee_datasets.js' %}" type="text/javascript"></script>
{% endblock %}

2. Implement Methods

In this step you'll implement the methods in the public/js/gee_datasets.js file that will update the options in the dataset controls dynamically. For example when the user selects a new Satellite Platform, the options of the Sensor select box will be updated to the sensors for that platform. The general approach will be to:

  • Create functions that update the controls with the currently saved values.

  • Save the value of a control anytime it changes.

  • Call the appropriate update functions when a control's value changes.

Here is a brief explanation of each method that will be implemented in this step:

  • update_sensor_options: updates the options of the sensor select box with options that correspond with the current satellite platform.

  • update_product_options: updates the options of the product select box with options that correspond with the current satellite platform and sensor.

  • update_date_bounds: updates the date range that is selectable for both date pickers based on the current product. The value of each date picker is also reset to fit within the new range if necessary.

  • bind_controls: used to bind the update methods to the change event of each control. Called when the module initializes after page load.

  • collect_data: used to collect the current values of all of the controls, as stored in our module, for use in our request for the map imagery later on.

  1. Replace the update_sensor_options method stub in public/js/gee_datasets.js with the following implementation:

update_sensor_options = function() {
    if (!m_platform in EE_PRODUCTS) {
        alert('Unknown platform selected.');

    // Clear sensor options

    // Set the Sensor Options
    let first_option = true;
    for (var sensor in EE_PRODUCTS[m_platform]) {
        let sensor_display_name = sensor.toUpperCase();
        let new_option = new Option(sensor_display_name, sensor, first_option, first_option);
        first_option = false;

    // Trigger a sensor change event to update select box
  1. Replace the update_product_options method stub in public/js/gee_datasets.js with the following implementation:

update_product_options = function() {
    if (!m_platform in EE_PRODUCTS || !m_sensor in EE_PRODUCTS[m_platform]) {
        alert('Unknown platform or sensor selected.');

    // Clear product options

    let first_option = true;

    // Set the Product Options
    for (var product in EE_PRODUCTS[m_platform][m_sensor]) {
        let product_display_name = EE_PRODUCTS[m_platform][m_sensor][product]['display'];
        let new_option = new Option(product_display_name, product, first_option, first_option);
        first_option = false;

    // Trigger a product change event to update select box
  1. Replace the update_date_bounds method stub in public/js/gee_datasets.js with the following implementation:

update_date_bounds = function() {
    // Get new date picker bounds for the current product
    let earliest_valid_date = EE_PRODUCTS[m_platform][m_sensor][m_product]['start_date'];
    let latest_valid_date = EE_PRODUCTS[m_platform][m_sensor][m_product]['end_date'];

    // Get current values of date pickers
    let current_start_date = $('#start_date').val();
    let current_end_date = $('#end_date').val();

    // Convert to Dates objects for comparison
    let date_evd = Date.parse(earliest_valid_date);
    let date_lvd = Date.parse(latest_valid_date) ? (latest_valid_date) :;
    let date_csd = Date.parse(current_start_date);
    let date_ced = Date.parse(current_end_date);

    // Don't reset currently selected dates if they fall within the new date range
    let reset_current_dates = true;

    if (date_csd >= date_evd && date_csd <= date_lvd && date_ced >= date_evd && date_ced <= date_lvd) {
        reset_current_dates = false;

    // Update start date datepicker bounds
    $('#start_date').datepicker('setStartDate', earliest_valid_date);
    $('#start_date').datepicker('setEndDate', latest_valid_date);
    if (reset_current_dates) {
        $('#start_date').datepicker('update', INITIAL_START_DATE);
        m_start_date = INITIAL_START_DATE;

    // Update end date datepicker bounds
    $('#end_date').datepicker('setStartDate', earliest_valid_date);
    $('#end_date').datepicker('setEndDate', latest_valid_date);
    if (reset_current_dates) {
        $('#end_date').datepicker('update', INITIAL_END_DATE);
        m_end_date = INITIAL_END_DATE;

    console.log('Date Bounds Changed To: ' + earliest_valid_date + ' - ' + latest_valid_date);
  1. Replace the bind_controls method stub in public/js/gee_datasets.js with the following implementation :

    bind_controls = function() {
        $('#platform').on('change', function() {
        let platform = $('#platform').val();

        if (platform !== m_platform) {
            m_platform = platform;
            console.log(`Platform Changed to: ${m_platform}`);
            // Update the sensor options when platform changes

    $('#sensor').on('change', function() {
        let sensor = $('#sensor').val();

        if (sensor !== m_sensor) {
            m_sensor = sensor;
            console.log(`Sensor Changed to: ${m_sensor}`);
            // Update the product options when sensor changes

    $('#product').on('change', function() {
        let product = $('#product').val();

        if (product !== m_product) {
            m_product = product;
            console.log(`Product Changed to: ${m_product}`);
            // Update the valid date range when product changes

    $('#start_date').on('change', function() {
        let start_date = $('#start_date').val();

        if (start_date !== m_start_date) {
            m_start_date = start_date;
            console.log(`Start Date Changed to: ${m_start_date}`);

    $('#end_date').on('change', function() {
        let end_date = $('#end_date').val();

        if (end_date !== m_end_date) {
            m_end_date = end_date;
            console.log(`End Date Changed to: ${m_end_date}`);

    $('#reducer').on('change', function() {
        let reducer = $('#reducer').val();

        if (reducer !== m_reducer) {
            m_reducer = reducer;
            console.log(`Reducer Changed to: ${m_reducer}`);
  1. Replace the collect_data method stub in public/js/gee_datasets.js with the following implementation:

collect_data = function() {
    let data = {
        platform: m_platform,
        sensor: m_sensor,
        product: m_product,
        start_date: m_start_date,
        end_date: m_end_date,
        reducer: m_reducer
    return data;
  1. Temporarily log the result of collect_data when the user clicks on the Load button to verify that everything is working correctly. Add the following to the bottom of the bind_controls method in public/js/gee_datasets.js:

$('#load_map').on('click', function() {
    let data = collect_data();

3. Test and Verify

Browse to http://localhost:8000/apps/earth-engine in a web browser and login if necessary. Verify the following:

  1. Open a JavaScript console in your web browser (in Chrome press CTRL-SHIFT-i or F12 and select the Console tab).

  2. Change the values of each control and note the output being logged to the console.

  3. The value of the each control that changes should be logged. For example, when the Satellite Platform control is changed, the Sensor, Product, and date controls should be updated.

  4. The Start Date control should not allow users to select dates before the beginning date of the selected dataset.

  5. Press the Load button and inspect the object that is logged to the JavaScript console. It should display the currently selected values of each control.

4. Solution

This concludes this portion of the GEE Tutorial. You can view the solution on GitHub at or clone it as follows:

git clone
cd tethysapp-earth_engine
git checkout -b dataset-controls-js-solution dataset-controls-js-solution-3.1